Screw-type dental implants and in particular externally threaded anchor pins for insertion into dental bone tissue of a patient to support artificial dental appliances, such as an artificial teeth, are well known in the art.
Common examples of such prior art devices are disclosed in U.S. Pat. Nos. 4,932,868 and 4,713,004, 5,061,181, 4,960,381, and 4,468,200.
Since dental anchor pins are generally of a substantially cylindrical shape intended to be completely submerged in bone tissue, with only an extreme upper end surface exposed to abut the prosthetic dental implant, the largest outer diameter of the anchor pin is generally the major thread diameter of the external threads on the pin. This allows a single bore-hole to be made in the patient's bone tissue of a diameter approximately equal to the minor thread diameter of the pin, thereby permitting the anchor pin and the external threads thereon to be fully screwably inserted into the bore-hole (see for example U.S. Pat. No. 4,960,381) in the patient's bone.
Frequently, however, the top portion of the dental pin is needed to both engage a tool to allow screwable insertion into the patient, and to also provide a mating surface for a prosthetic implant, such as an artificial tooth.
Accordingly, the top portion of the pin frequently of necessity needs to be of a diameter larger than the bore-hole diameter, which as mentioned previously, is drilled to the minor thread diameter of the pin. Such a configuration requires countersinking of the bore-hole if the top portion of the pin is to fit into the bone and to be submerged level with the bone tissue of the patient without interference. Countersinking must accordingly be done either by drilling a second larger bore for a portion of the bore-hole, or using a double-sized drill bit, as is disclosed in U.S. Pat. No. 5,061,181, to thereby provide a bore of two diameters.
For example, FIG. 2 of U.S. Pat. No. 5,061,181 (FIG. 1 of the drawings herein) discloses an anchor pin 2, having a top portion 17 of diameter D.sub.z which is larger than the minor thread diameter D.sub.x of threads 46. Such a configuration thereby requires a countersinking operation (i.e. an additional wider bore for a portion of the bore) if the circumferential ridges 39 and top portion 17 of the anchor pin are to be submerged within a drilled bore of diameter D.sub.x.
Apart from prolonging the duration of an operation on a patient by requiring the drilling of a second bore, countersinking has an additional drawback in that countersinking removes additional dental bone from the mouth of the patient. This ground bone, because of its cortical nature, is extremely useful in anchoring the dental implant, as it can greatly speed and assist in bone growth around the implant. Accordingly, it is very desirable that the ground bone and bone chips be retained in the bore if at all possible to assist in autogenous rapid re-growth of bone around the prosthetic anchor pin to thereby assist in anchoring the pin within the patient's bone tissue.
Recently-developed anchor pins, such as the anchor pin 10 disclosed in both U.S. Pat. No. 4,713,004 and 4,932,860 (see FIG. 2 thereof and FIG. 2A and 2B of the drawings herein) make use of this concept. For example, U.S. Pat. No. 4,713,004 discloses channels 18 formed in anchor pin 10 which extend through the threads 13, 13' on the body of the anchor pin 10. Threads 13 along one side edge of the channel 18 are disposed at a right angle to the circumferential direction of rotation A of the pin 10 while the threads 13' (see FIG. 2 of the drawings herein) on the other side edge of the channel 18 are at an oblique angle to the circumferential direction of rotation A of the pin 10. This allows the right-angled edge of threads 13 to shave off bone chips during threading of the pin into the bone, and to direct the pieces of bone into the channel 18 and vent 16, so that pieces of bone are directed to the base portion of the bore-hole and the pin to assist in bone growth in this area, thereby improving the anchoring of the pin 10 to the patient's bone.
Notably, however, the diameter D.sub.z of the top portion of the pin 10 is typically greater than the minor diameter of the threads D.sub.x, particularly where a sufficient mating surface area is required to accommodate a hex nut and at the same time provide a sufficient mating surface for the artificial tooth (see for example the prior art anchor pin 10 shown in FIG. 2A and 2B herein and U.S. Pat. No. 4,713,004). Thus countersinking is necessary and should be done if the top portion of the pin 10 is to be fully submerged.
If countersinking is not carried out, although the self-tapping threads 13 remove some of the bone material when the pin is threadably inserted into the patient, the top portion 16 of the anchor pin 10 because of its larger diameter DZ will be forcibly compressed against bone tissue surrounding the smaller diameter bore-hole of diameter Dx, upon full insertion of the pin 10. This has been found to be extremely undesirable, as it tends to strip the threaded bone tissue, and in addition because of the larger diameter it tends to be compressed against the bone thereby impeding circulation in the bone tissue surrounding the top portion 16 of the implant (pin), resulting poor bone structure surrounding the implant, and thus a much weaker dental implant. As well, valuable bone chips for the area of the bore displaced by the top portion 16 of the pin 10 are not utilized nor displaced elsewhere to other locations proximate the pin 10 to assist in reestablishing bone growth around the anchor pin 10.
Accordingly, despite the prior art, a real and substantial need exists for a dental anchor pin which has a top portion of a diameter greater than the minor thread diameter of the pin, but does not require a separate countersinking operation. A real need further exists for an anchor pin that is able to self-countersink and utilize and relocate bone chips from the self-countersinking operation to assist in autogenous regeneration of bone at same or other locations around the prosthetic anchor pin.